The gold-bearing reefs of the Transvaal present a good illustration.
Beds of conglomerate consisting chiefly of quartz and quartzite
pebbles have experienced crushing and shattering, and have had
their natural porosity, much enhanced by these after-effects.
Solutions of gold, coming through, have encountered pyrites and
have had the gold precipitated upon the pyrites, which is itself
often broken and granulated. In other regions shearing has led to
sheeting and opening of the rocks by many parallel cracks but
almost always with such marked displacement that the next type
most correctly describes them. From any point of view the shear-zone
is a natural transition to the fault and closely related to it.
F. Deposits in Faults.—This type of ore-body was one of the earliest established, and has always figured very prominently in the minds of students of the subject since the first systematic formulations of our knowledge. The dislocation of the earth’s crust by faults has furnished either clean-cut fissure or else lines of closely set parallel fractures, whose combined displacement has been comparatively great. The faults go to relatively profound depths and they furnish therefore waterways of extended character, which may reach from regions of heat and pressure in depth to regions of cold and diminishing pressure above; thus from conditions favourable to solution below to conditions favouring precipitation toward the surface. Faults often occur, moreover, in connexion with eruptive outbreaks, and therefore in circumstances especially favourable to ore deposition. From all these reasons it is not surprising that the “true fissure vein” based on a profound fault has been the ideal of the prospector’s search in many parts of the world, and has often been his reward. The historic veins of Cornwall and of Saxony are of this type, also the great silver veins of Mexico, the gold veins of California, the great silver-gold deposits of the Comstock lode, and many in South America.
Faulting often leads to great shattering of the country rock, and instead of being a clean-cut open cavity, there results a brecciated belt which may then be cemented by infiltrating ore and gangue. In the midst of this the richer ore occurs as bonanzes or chutes, which are succeeded by leaner stretches. The movement of the walls produces the polished surfaces specifically called “slicken-sides,” parallel to which the ore-chutes often run. The change in the character of the entering solutions from time to time gives a banded character to the deposit, so that from both walls toward the centre corresponding layers succeed one another. At the centre the last layers may meet as interlocking crystals in the familiar comb-in-comb structure or they may leave cavities called “vugs” into which beautiful and perfectly formed crystals project (see fig.). Fault fissures swell and pinch affording wide and narrow places in the resulting ore-body. They often intersect each other and one may throw or heave another, according to the mechanics of faulting as set forth under the article on Geology.
While fault-fissures have in no way failed in later years to be appreciated by mining geologists, yet they do not hold that predominant place which in the days of more limited experience was theirs. On the contrary, other types such as contact zones, replacements and impregnations are found to be of scarcely inferior importance. Nevertheless the last two, at least, must usually owe to the fault-fissure the waterway which has brought in the solutions.
A very peculiar non-metallic deposit found in fault-fissures and imitating the ordinary veins in all essentials is furnished by the asphaltic minerals, often described as asphaltic coals and known in mineralogy as “grahamite,” “albertite,” “uintaite,” “gilsonite,” &c. Petroleums with asphaltic bases have percolated into fault-fissures and have there deposited on evaporation and oxidation their dissolved burdens. The black coaly mineral presents all the geological relations of a fissure vein and is mined like so much ore.
G. Volcanic Necks.—A very unusual ore-body is furnished by this type, which is only known in a few instances. In two mines, however, in Colorado, the Bassick and the Bull-Domingo, there occur chimneys of elliptical cross-section filled with rounded boulders, and believed with much reason to be the tubes of small explosive volcanoes. After brief periods of activity they became waterways for uprising heated solutions which filled the interstices with ore.
III. Deposited from Suspension.—The ores which result from this process are all formed upon the surface of the earth and through the action of water. They are primarily the result of the weathering of rocks and of the removal of the loose products thus afforded in the ordinary processes of erosion.
A. Placers.—Many useful minerals, including some of a metallic character, are very resistant to the agents of decomposition which cause the disintegration of the common rocks. Thus magnetite is a mineral present in a minor capacity in all eruptives and in fairly large percentage in many of the basic types. It is proof against protracted exposure to natural reagents, and it is heavy. Becoming freed by the disintegration of the containing rock it is mingled with the transported materials of running streams, and settles with other heavy minerals wherever the current slackens to a sufficient degree. Concentration may thus ensue and beds of black sand result. If again deposits of loose sand containing more or less magnetite are exposed to the surf of the ocean, or even to the waves of lakes, a similar sorting action takes place on the beach. The magnetite remains behind while the undertow removes the lighter materials. Iron sands of either of these varieties are usually too rich in titanium to be of commercial value, but with the magnetite may be gold or platinum in sufficient amount to be of value.
While magnetite is the commonest of the ores to be found in placers, gold is the metal which usually gives them value. Wherever systems of drainage have eroded gold-bearing rocks, the gold has passed into the streams with the other detrital materials, and, even though in very fine flakes, being yet very heavy has sunk to the bottom in the slackened water and has there enriched the gravel. The gold tends to work its way through the gravels even to the bed-rock, or to some bed of interstratified and impervious clay, and there to be relatively rich. It favours also the insides of bends and the heads of quiet reaches. When a small tributary stream joins a larger one and is both checked itself and checks the current of the large one, the gold, as in the Klondike, tends to settle in relatively great abundance.
Pot-holes, strangely enough, or related rock-cavities, often fail to yield the nuggets, apparently because the swirl of the water and grit has ground them to impalpable powder. The particles have then been washed elsewhere.
When the gold-bearing gravels are panned down a small residue is obtained of all the heavy minerals in the gravel. Magnetite is the commonest and gives the technical name of “black sand” to the concentrate. With it, however, there are almost always found garnet and other less familiar minerals. If the stream valley has been hunted over by sportsmen with shot-guns or rifles, the lost shot and bullets are commonly caught in the pan. Even diamonds have been rarely noted and they may, indeed, be specially sought in gravels.
Along sea-beaches where great beds of auriferous gravel have been attacked by the surf, concentrated bars carrying nuggets and flakes of gold in workable quantity have not infrequently resulted. Cape Nome, Alaska, is perhaps the most productive of all. The gold in the beach-placers is usually worn by the constant attrition into extremely fine particles, and the flakes or colours are more difficult to save than in the case of stream-placers.
In some regions of gold-bearing rocks, as in the south-eastern United States, the products of superficial decay of rocks may remain in situ and be sufficiently charged with gold to be washed for the yellow metal. They are different from the usual placer deposit although hydraulicked in the same way. They might be properly considered residual deposits under the next head.
Auriferous stream-gravels of ancient and long-abandoned systems of drainage may remain beneath lava flows or later sedimentary accumulations and be the objects of underground mining. Both in Australia, where they are called “deep leads,” and in California, where they are called “buried channels” or “deep gravels,” they have been for many years the objects of mining. In California the bed-rock is usually slate or schist and a series of technical terms have resulted descriptive of the rich streaks. The bed-rock is called the rim-rock; the pay-streaks which appear on its sides, bench-gravels, and the lowest one the channel-gravel. Tunnels are often very skilfully driven through the rim-rock to strike the channel-gravel and at the same time preserve the proper slope for drainage and extraction. The buried channels in California have proved of much scientific interest from the remains of prehistoric man, skulls, mortars and pestles which they have yielded.
Among the non-metallic minerals sought from placers, phosphates for fertilizers hold a position of great importance.
B. Residual Deposits.—As contrasted with the placers whose materials are derived by transport from a distance, we sometimes find heavy and resistant minerals, once contained in the rock but freed by the process of decay and disintegration. The lighter loose materials are washed away and deposited elsewhere. The heavy remain behind in a concentrated condition. Iron ores of this character are known, and chromite is set free in the same way by the decomposition of serpentine.
In the decay of ferruginous rocks like limestones the iron may be changed to the insoluble ferric hydrate, brown hematite, and remain as veinlets and crusts throughout a mantle of clay. The brown hematite may be freed by artificial washing and used as an iron ore.
IV. Carbonaceous Deposits from Vegetation.—Far the most important of the non-metallic minerals are those composing the coal series. They yield entire strata analogous to other sedimentary rocks, but in most cases from vegetation which has grown in situ. They are found in all stages from nearly carbonized leaves and woody tissue in peat, through much more altered materials in lignite and bituminous coal to extremes in anthracite and graphite. The prime necessity for their preservation from decay is furnished by water, in or near which they must grow, and beneath which they must be deposited, so that oxidation may be retarded. In instances they have been heaped together by rivers, especially when at flood. The method of origin is fully discussed under Coal and under
